Abstract: Many clinical isolates of Staphylococcus aureus (S. aureus) are resistant to numerous antimicrobials, including the fluoroquinolones (FQs). Flavonoids such as biochanin A (BCA) are compounds that are naturally present in fruits, vegetables, and plant-derived beverages. The goal of this investigation was to study the possible synergy between the antimicrobial agents BCA and ciprofloxacin (CPFX) when used in combination; CPFX was chosen as a representative FQ compound. We used S. aureus strain ATCC 25923 and 11 fluoroquinolone (FQ)-resistant methicillin-resistant S. aureus (MRSA) strains. Results from the drug susceptibility testing and checkerboard assays show that the minimum inhibitory concentration (MIC) of BCA ranged from 64 µg/mL to 512 µg/mL. When BCA was combined with CPFX, the fractional inhibitory concentration index (FICI) data showed that there was synergy in all 12 of the S. aureus strains tested. No antagonistic activity was observed in any of the strains tested. The results of time-kill tests and agar diffusion tests confirm that there was synergy between BCA and CPFX against S. aureus strains. These results suggest that BCA can be combined with FQs to produce a powerful antimicrobial agent.

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